1. Field of the Invention
The present invention relates to atomic force microscopy and, more specifically, to an atomic force microscopy system employing a membrane-type actuator.
2. Description of the Prior Art
Existing atomic force microscopes (AFM) have been used to probe a wide range of physical and biological processes, including mechanical properties of single molecules, electric and magnetic fields of single atoms and electrons. Moreover, cantilever based structures inspired by the AFM have been a significant driver for nanotechnology resulting in chemical sensor arrays, various forms of lithography tools with high resolution, and terabit level data storage systems. Despite the current rate of success, the AFM needs to be improved in terms of speed, sensitivity, and an ability to generate quantitative data on the chemical and mechanical properties of the sample. For example, when measuring molecular dynamics at room temperature, the molecular forces need to be measured in a time scale that is less than the time of the thermal fluctuations to break the bonds. This requires a high speed system with sub-nanonewton and sub-nanometer sensitivity.
Current cantilever-based structures for AFM probes and their respective actuation methodologies lack speed and sensitivity and have hindered progress in the aforementioned areas. Imaging systems based on small cantilevers have been developed to increase the speed of AFMs, but this approach has not yet found wide use due to demanding constraints on optical detection and bulky actuators. Several methods have been developed for quantitative elasticity measurements, but the trade-off between force resolution, measurement speed, and cantilever stiffness has been problematic especially for samples with high compliance and high adhesion. Cantilever deflection signals measured during tapping mode imaging have been inverted to obtain elasticity information with smaller impact forces, but complicated dynamic response of the cantilever increases the noise level and prevents calculation of the interaction forces. Arrays of AFM cantilevers with integrated piezoelectric actuators have been developed for parallel lithography, but low cantilever speed and complex fabrication methods have limited their use.
Moreover, conventional methods of imaging with scanning probes can be time consuming while others are often destructive because they require static tip-sample contact. Dynamic operation of AFM, such as the tapping-mode, eliminates shear forces during the scan. However, the only free variable in this mode, the phase, is related to the energy dissipation and it is difficult to interpret. Further, the inverse problem of gathering the time-domain interaction forces from the tapping signal is not easily solvable due to complex dynamics of the AFM cantilever. Harmonic imaging is useful to analyze the sample elastic properties, but this method recovers only a small part of the tip-sample interaction force frequency spectrum.
Therefore, there is a need to overcome these and other problems of the prior art associated with probe microscopy.